In anaesthesia, the gaseous anaesthetic agent inhaled by the patient is formed of oxygen, nitrogen, nitrous oxide and an inhalation anaesthetic. Inhalation anaesthetics are typically in liquid form at administration temperatures, and an anaesthetic vaporizer is needed to gasify them. Anaesthetic vaporizers have a liquid container for vaporizing the liquid. The vaporized anaesthetic is further administered for the patient to inhale by means of a carrier gas flow.
Vaporizer containers are provided with flow conduits and valves by way of which inhalation liquid may be added to the container or, when necessary, drained therefrom. Liquid is added and withdrawn either by means of a filling device specific to the anaesthetic or by directly pouring out into a filling hopper. It is characteristic of the filling device that it can only be fixed to the transport bottle for the given anaesthetic liquid and to the vaporizer. The filling device incorporates a liquid flow conduit and a gas flow conduit. When the container is being filled, liquid flows out from the bottle into the container via the flow conduit, and an equivalent volume of gas flows from the container into the bottle, thus preventing subatmospheric pressure from being produced. When the container is emptied, the procedure is reverse.
It is essential for the operation of the vaporizer that the container is not filled over the maximum limit. If the container is overfilled, this may result, depending on the extent of overfilling, either in an overly high anaesthetic content, which may in the worst case rapidly become lethal, or in stopping of the vaporizing, which may cause the patient to awaken too early.
In prior art constructions, anaesthetic containers are provided with overfill protectors automatically cutting off the flow in the gas flow conduit when the liquid container is full. Thus a subatmospheric pressure is produced in the bottle, the liquid cannot flow into the container, and filling is stopped. The above implements are so constructed that the outlet for the gas flow conduit in the liquid container is positioned at the maximum liquid level. Once the liquid has reached this level, either the gas conduit is filled with liquid, thus cutting off the flow, or a valve shutting automatically at the maximum liquid level is provided in the outlet for the gas flow conduit which is on the liquid container side. Such a valve may be e.g. a float in the liquid, rising with the liquid level until it reaches the outlet and shuts the outlet, thus preventing any gas flow through the gas flow conduit from the container. In present-day anaesthetic vaporizers, the above filling device and overfilling protection mechanism is provided in the wall of the liquid container.
The drawback with the prior art arrangements is that in order for them to operate correctly, the liquid container must be correctly positioned. This means that the vaporizer must typically have an upright position. Hence instructions for filling are provided in operator's instructions for the apparatus. If the vaporizer is filled in an inclined position in such a way that the end of the liquid container facing the filling device is located at a higher level, the liquid level at the end facing the filling device is correspondingly lower, with the result that the overfill protector will not be operative until the liquid container is overfilled.